In fact, 42 percent of women have more than $30,000 in college debt, compared with 27 percent of men. Women also are two times more likely than men to think it will take more than 20 years to pay off their loans, according to market research firm ORC International.

While there are many factors that contribute to a decision to postpone children, including changing attitudes about age and motherhood, access to birth control and increased opportunities in the workforce, student loans are increasingly to blame.

In a March poll by Future Family, a start-up that helps women understand fertility, 44 percent of women said they had student debt. Half of those women said the loans affect their decision about having children. Future Family polled nearly 1,000 women in the U.S. who were ages 25 to 40 and didn't have children.

"We find that concerns about finances — and student debt specifically — are factoring in more," said Claire Tomkins, Future Family's CEO. "It ends up being a financial equation that's a little untenable."